Photon down-conversion in Terbium(III)-doped thin dielectric films and fluorozirconate glasses for thin film solar cells

The efficiency of thin film solar cells can be improved with the addition of a photon down-conversion top layer. This layer converts incident ultraviolet light of the solar spectrum to visible light, which transmits through the glass and is efficiently absorbed by the active layer of the solar cell. The results of our investigations of thin dielectric films and fluorozirconate glass, both doped with Tb3+ ions, are presented. Tb3+ has absorption bands between 250 and 380 nm; the corresponding emission bands are in the spectral range between 400 and 630 nm. Thin SiO2 and Al2O3 films with 0.04 - 10.18 at.% Tb were prepared by co-sputtering. For both as-deposited film systems, the highest fluorescence intensity is found for a Tb3+ doping level of approximately 1 at.%; the fluorescence intensity of Tb3+ in SiO2 is higher than that in Al2O3. Thermal treatment leads to an enhancement of the fluorescence intensity by more than one order of magnitude and the highest fluorescence intensity is found for 2 at.% Tb for annealed thin SiO2 films containing Tb3+. For comparison, the absorption and emission properties of Tb3+-doped fluorozirconate glass are investigated for a doping level of 0.3 at.% Tb.